1. Field of the Invention
The present invention relates to an image detector having a recording-side photoconductive layer, a reading-side photoconductive layer, and stripe electrodes. In the image detector, an electrostatic latent image is temporarily recorded by applying recording light to the recording-side photoconductive layer, and the recorded electrostatic latent image is reading out through the stripe electrodes by applying reading light to the reading-side photoconductive layer.
2. Description of the Related Art
Conventionally, various apparatuses such as facsimile apparatuses, copiers, radiographic imaging apparatuses use image detectors. Some radiographic imaging apparatuses designed for medical applications use as an image detector an optical-reading type solid-state radiographic image detector (or an optically readable electrostatic recording medium which records radiographic images), which comprises a photoconductive body (layer) made of a material exhibiting conductivity in response to exposure to radiation such as X rays. For example, the photoconductive body (layer) is a selenium plate. In the above radiographic imaging apparatuses, recording electromagnetic radiation (which may be called recording light) is applied to the solid-state radiographic image detector, so that charges having a polarity (hereinafter called a latent-image polarity), out of the charges (pairs of opposite charges) generated in the photoconductive body (layer) by the exposure to the recording electromagnetic radiation, are stored as latent-image charges in a charge storage region of the solid-state radiographic image detector, and the amount of the latent-image charges stored in each area (corresponding to a pixel) of the charge storage region corresponds to the exposure dose of the recording electromagnetic radiation in the area. Thus, radiographic image information is recorded in the form of a latent image. Thereafter, a reading-side electrode layer of the solid-state radiographic image detector is scanned with reading electromagnetic radiation (which may be called reading light) so that the amount of signal charges corresponding to the latent-image charges recorded in each area of the solid-state radiographic image detector is detected in the form of an electric signal (current). Thus, the recorded radiographic image information is read out. Typically, the above recording electromagnetic radiation is realized by X rays, and the above reading electromagnetic radiation is realized by a laser beam or a line-shaped light band. The above radiographic imaging technique is disclosed in U.S. Pat. No. 5,268,569, International Patent Publication WO-A1-98/59261, and Japanese Unexamined Patent Publication Nos. 9(1997)-5906, 2000-162726, 2000-284056, and 2000-284057. The contents of the above patent publications are incorporated by reference in the present patent application.
In particular, the Japanese Unexamined Patent Publication Nos. 2000-162726, 2000-284056, and 2000-284057 disclose solid-state radiographic image detectors which are constructed by forming a recording-side electrode layer (first electrode layer), a recording-side photoconductive layer, a charge transport layer, a reading-side photoconductive layer, and a reading-side electrode layer (second electrode layer) in this order so that a charge storage region is realized between the recording-side photoconductive layer and the charge transport layer. The recording-side electrode layer (first electrode layer) is transparent to recording light. The recording-side photoconductive layer generates charges and exhibits conductivity when the recording-side photoconductive layer is exposed to the recording light which has passed through the recording-side electrode layer. The charge transport layer behaves as almost an insulator against charge carriers having the latent-image polarity (i.e., the same polarity as the latent-image charges), and behaves as almost a conductor of charge carriers having the opposite polarity to the latent-image polarity (which is hereinafter called a transport polarity). The charges having the transport polarity are called transport charges. The reading-side photoconductive layer generates charges and exhibits conductivity when the reading-side photoconductive layer is exposed to reading light. The reading-side electrode layer (second electrode layer) is transparent to the reading light. When the reading light is applied to the reading-side photoconductive layer through the reading-side electrode layer, the electric signal corresponding to the amount of the latent-image charges stored in each area of the charge storage region is detected through the reading-side electrode layer.
In addition, the Japanese Unexamined Patent Publication Nos. 2000-162726, 2000-284056, and 2000-284057 disclose techniques for detecting the amount of signal charges. According to the disclosed techniques, the reading-side electrode layer includes a striped (or comb) electrode array comprised of a number of linear electrodes which are elongated in the feeding direction in the scanning of the reading-side photoconductive layer with the reading light, and arranged parallel to each other. The linear electrodes are respectively connected to detection amplifiers. The reading light has a cross section of a line shape elongated in the main scanning direction, which is perpendicular to the feeding direction, and is moved in the feeding direction for scanning the entire area of the reading-side photoconductive layer through the reading-side electrode layer. The above technique for detecting the amount of signal charges is called a line-reading-out method.
According to the above line-reading-out method, the amounts of signal charges corresponding to pixels of the reading-side photoconductive layer located on each line in the main scanning direction are concurrently read out. Therefore, the reading speed can be increased. In addition, since the reading-side electrode layer is divided into the linear electrodes, the distributed (load) capacitance of each detection amplifier decreases, and therefore the S/N ratio can be increased. Further, since the positions in which the latent-image charges are stored can be fixed to the positions in which the linear electrodes are arranged, the structural noise can be reduced. That is, the line-reading-out method has various advantages.
Further, the Japanese Unexamined Patent Publication Nos. 2000-284056 and 2000-284057 disclose an image detector in which linear charging electrodes (linear charge-read-out electrodes) are arranged parallel to the linear electrodes constituting the striped electrode array so that the linear charging electrodes can be used in the operation of detecting the amount of the latent-image charges in the form of the electric signal. Hereinafter, the linear electrodes constituting the striped electrode array may be called light-entrance electrodes.
When the linear charging electrodes are arranged as above, additional capacitors are formed between the charge storage region and the respective linear charging electrodes, and it is therefore possible to store the transport charges in the linear charging electrodes by charge rearrangement before reading out the electric signal corresponding to the amount of the latent-image charges, where the transport charges have the opposite polarity to that of the latent-image charges stored in the charge storage region by the recording. Therefore, the amounts of the transport charges distributed to the capacitors which are formed between the charge storage region and the light-entrance electrodes can be decreased by the provision of the linear charging electrodes. Accordingly, the amount of signal charges detected by the image detector can be increased, and thus the readout efficiency can be increased. Further, the above advantage of the provision of the linear charging electrodes and the advantages (e.g., the great responsiveness) of the provision of the striped electrode array can coexist.
Furthermore, when the transmittance of the reading light through the light-entrance electrodes is small, the amount of the reading light entering the reading-side photoconductive layer becomes insufficient. In addition, when the transmittance of the reading light through the linear charging electrodes is great, the linear charging electrodes also function as light-entrance electrodes, and the amount of the signal charges detected through the linear charging electrodes may decrease. The commonly-assigned U.S. patent application Ser. No. 09/620,707 corresponding to Japanese patent application Nos. 11 (1999)-207283 and 2000-209529 discloses a condition of transmittances and widths of the linear charging electrodes and the light-entrance electrodes for making the amount of light entering the reading-side photoconductive layer through the light-entrance electrodes greater than the amount of light entering the reading-side photoconductive layer through the linear charging electrodes, and substantially increasing the readout efficiency.
However, even when the transmittances and widths of the linear charging electrodes and the light-entrance electrodes satisfy a predetermined condition for preventing light-induced discharge (light readout) in the mid-width portion of each linear charging electrode, the light-induced discharge is likely to occur in the near-edge portions of each linear charging electrode since the electric field concentrates in the near-edge portions due to the edge effect. Therefore, the readout efficiency can decrease even when the transmittances and widths of the linear charging electrodes and the light-entrance electrodes satisfy the above predetermined condition. In order to decrease the probability of occurrence of the light-induced discharge, thicknesses of the near-edge portions of each linear charging electrode can be increased, or the near-edge portions of each linear charging electrode can be rounded off. However, in this case, the manufacturing process becomes more complex, and the manufacturing cost increases.
In order to satisfy the above predetermined condition, the light-entrance electrodes and the linear charging electrodes must be made of different materials. Therefore, the construction of the electrodes becomes complex. In addition, it is not easy to form electrodes with different materials within the same layer. Further, when electrodes of different materials are formed within the same layer, the manufacturing process becomes more complex, and the manufacturing cost further increases.
An object of the present invention is to provide an image detector which comprises light-entrance electrodes and linear charging electrodes, and can substantially increase the readout efficiency.
Another object of the present invention is to provide an image detector which comprises light-entrance electrodes and linear charging electrodes, can substantially increase the readout efficiency, and is easy to produce.
According to the present invention, there is provided an image detector comprising a recording-side photoconductive layer, a reading-side photoconductive layer, a charge storage region, and a pair of electrode layers. The recording-side photoconductive layer generates latent-image charges and exhibits conductivity when the recording-side photoconductive layer is exposed to first electromagnetic radiation for recording an image. The reading-side photoconductive layer generates charges and exhibits conductivity when the reading-side photoconductive layer is exposed to second electromagnetic radiation for reading an image. The charge storage region is formed between the recording-side photoconductive layer and the reading-side photoconductive layer, and stores the latent-image charges. The pair of electrode layers are arranged to sandwich the recording-side photoconductive layer and the reading-side photoconductive layer, and apply an electric field to the recording-side photoconductive layer and the reading-side photoconductive layer. One of the pair of electrode layers located near to the reading-side photoconductive layer comprises a plurality of first linear electrodes which are transparent to the second electromagnetic radiation, and arranged parallel to each other, and a plurality of second linear electrodes each of which corresponds to at least one of the plurality of first linear electrodes, is arranged parallel to the at least one of the plurality of first linear electrodes, and outputs an electric signal corresponding to the amount of the latent-image charges when the at least one of the plurality of first linear electrodes is scanned with the second electromagnetic radiation. The image detector further comprises means for making a first strength of irradiation of each of the plurality of second linear electrodes with the second electromagnetic radiation smaller than a second strength of irradiation of at least one of the plurality of first linear electrodes corresponding to the second linear electrode with the second electromagnetic radiation.
In the above description of the present invention, the term xe2x80x9celectromagnetic radiationxe2x80x9d is used in its broadest sense, and includes light, X rays, gamma rays, and any other electromagnetic waves having shorter or longer wavelengths.
Since the above means is provided in the image detector according to the present invention, the strength of irradiation of the plurality of second linear electrodes (which are provided for reading out the electric signal corresponding to the amount of the latent-image charges) with the second electromagnetic radiation (i.e., reading light) can be decreased, and it is therefore possible to avoid the decrease in the readout efficiency caused by the edge effect produced in the near-edge portions of each of the plurality of second linear electrodes. In addition, the above decrease in the strength of irradiation of the plurality of second linear electrodes can be achieved without specially processing the first and second electrodes. Therefore, the manufacturing process does not become complex, and the manufacturing cost does not increase.
Preferably, the image detector according to the present invention also has one or any possible combination of the following additional features (i) to (v).
(i) The above means may be realized by a plurality of shading films arranged to shade the plurality of second linear electrodes from the second electromagnetic radiation. Since, in this case, the plurality of second linear electrodes are shaded from the second electromagnetic radiation, the plurality of first linear electrodes and the plurality of second linear electrodes can be made of an identical material, and thus the image detector which achieves high readout efficiency can be manufactured easily.
(ii) The first strength and the second strength may satisfy a relationship, Ub/Ucxe2x89xa75, where Ub represents the first strength, and Uc represents the second strength. More preferably, the first strength and the second strength satisfy a relationship, Ub/Ucxe2x89xa78. Further preferably, the first strength and the second strength satisfy a relationship, Ub/Ucxe2x89xa712.
(iii) In the image detector having the feature (i), when each of the plurality of second linear electrodes has a width Wc, each of the shading films has a width Wd, and a gap Wbc exists between each second linear electrode and each of at least one of the plurality of first linear electrodes corresponding to the second linear electrode, the width Wc, the width Wd, and the gap Wbc may satisfy a condition,
Wcxe2x89xa6Wdxe2x89xa6(Wc+2xc3x97Wbc).xe2x80x83xe2x80x83(1)
This condition indicates that each of the shading films completely covers the corresponding one of the plurality of second linear electrodes, and a gap corresponding to at least the width Wb of each of the plurality of first linear electrodes is secured between adjacent electrodes of the shading films so that the reading light can pass through the gap, and the full width of each of the plurality of first linear electrodes is exposed to the second electromagnetic radiation.
(iv) In the image detector having the feature (i), when each of the plurality of second linear electrodes has a width Wc, each of the shading films has a width Wd, and a gap Wbc exists between each second linear electrode and each of at least one of the plurality of first linear electrodes corresponding to the second linear electrode, the width Wc, the width Wd, and the gap Wbc may satisfy a condition,
(Wc+Wbc/2)xe2x89xa6Wdxe2x89xa6(Wc+Wbc).xe2x80x83xe2x80x83(2)
(v) When each of the plurality of first linear electrodes has a width Wb and a transmittance Pb for the second electromagnetic radiation, and each of the plurality of second linear electrodes has a width Wc and a transmittance Pc for the second electromagnetic radiation, the widths Wb and Wc and the transmittances Pb and Pc may satisfy a condition,
(Wbxc3x97Pb)/(Wc+Pc)xe2x89xa75.xe2x80x83xe2x80x83(3)
More preferably, the widths Wb and Wc and the transmittances Pb and Pc satisfy a condition,
(Wbxc3x97Pb)/ (Wc+Pc)xe2x89xa78.
Further preferably, the widths Wb and Wc and the transmittances Pb and Pc satisfy a condition,
(Wbxc3x97Pb)/(Wc+Pc)xe2x89xa712.
When one of the above condition (3), the more preferable condition, and the further preferable condition is satisfied, the readout efficiency can be further increased.
The charge storage region can be formed as follows.
(a) The charge storage region can be formed at the boundary between the charge transport layer and the recording-side photoconductive layer, as disclosed in the coassigned U.S. patent application, Ser. No. 09/404,371 (and the corresponding Japanese Unexamined Patent Publication No. 2000-162726) and the coassigned U.S. patent application, Ser. No. 09/539,412 (and the corresponding Japanese Unexamined Patent Publication No. 2000-284056).
(b) A trap layer may be provided. In this case, the charge storage region can be formed in the trap layer or at the boundary between the trap layer and the recording-side photoconductive layer, as disclosed in the U.S. Pat. No. 4,535,468.
(c) Microplates (minute conductive members) may be provided for collecting and storing the latent-image charges, as disclosed in the coassigned U.S. patent application, Ser. No. 09/538,479 (and the corresponding Japanese Unexamined Patent Publication No. 2000-284057).
A radiographic image can be recorded in and read out from the image detector according to the present invention by using the conventional recording and reading methods and circuits, for example, as explained in the coassigned U.S. patent application, Ser. No. 09/538,479 (and the corresponding Japanese Unexamined Patent Publication No. 2000-284057).